The primary long-term goal of this project is the achievement of a further understanding of the metabolism and regulation of red cell formation in the fetus. This investigation will include several aspects of fetal erythropoiesis, among them, specific internal fetal control mechanisms, the placental transfer and the role of the nutritive elements, iron and vitamin B12, and influences related to the status of hematologic and other vital systems within the mother. Studies will be conducted in pregnant dogs, sheep and monkeys. Fetuses will be prepared in utero with vascular catheters and then maintained chronically for periods of days or weeks. Parameters of fetal hematologic function will be determined in normal fetuses and in fetuses undergoing specific alterations in hematologic status. Fetuses with chronically indwelling vascular catheters will also be used in the investigation of the placental transport of iron and vitamin B12.